This invention relates generally to ball nuts such as are employed in combination with a ball screw or the like to precisely position a moving member such as a machine tool axis slide along a fixed member such as a machine tool base, and more specifically, this invention relates to a ball nut having adjustable preloading.
In the fabrication of various machines, specifically numerically controlled machine tools, it is often desirable to provide for rapid and accurate positioning of a movable member, such as an axis slide in the case of a machine tool, on a fixed member, such as a machine tool bed. The most common means employed for providing rapid and accurate positioning of a movable member on a fixed member are the well known ball screw and ball nut. Typically, the ball screw is journaled into the fixed member parallel to the desired path of movable member movement and the ball nut is affixed to the movable member so as to be in threaded engagement with the ball screw. In operation, the ball screw is rotatably driven by a servo controlled motor in response to numerical control commands to displace the ball nut therealong, thereby precisely positioning the movable member along the fixed member.
Heretofore, when the combination of a ball screw and ball nut have been employed in a machine tool or the like to precisely position a movable member along a fixed member, the ball nut has been affixed to the movable member so that the loading on the ball nut remains fixed during the useful life of the ball nut. Generally, fixing the preload on the ball nut during machine tool fabrication incurs no difficulty during subsequent machine tool use since present day ball screw velocities are low so that fixing ball screw preload to make the ball nut relatively stiff does not place any undue strain on the ball screw. In fact, fixing the ball nut preload to make the ball nut very stiff axially is usually desirable at low ball screw velocities because of the relatively large forces on the ball screw.
The advent of very high speed spindles capable of performing cutting operations at 20,000 rpm and above and the advent of very durable tooling has made present day machine tool feedrates, that is, machine tool axis slide velocities of up to 10 meters/minute (400 inches per minute) too slow to obtain maximum machine tool efficiency during certain machining operations on certain types of material. To obtain maximum machine tool efficiency under such conditions may require feedrates of 100 meters/minute or higher. At such high feedrates and by implication, such high ball screw speeds, it is imperative that ball screw drag be reduced, which is best accomplished by decreasing ball nut preloading as the forces on the ball screw at such high speeds are generally low in comparison to the forces on the screw at low screw speeds. Thus, it is desirable to adaptively vary the preloading on the ball nut in relation to ball screw speed. Heretofore, this has not been possible due to the fact that ball nut preloading was fixed.
To overcome the difficulties attributed to the fixed ball nut preloading, the present invention provides a ball nut whose preloading is adjustable to facilitate large preloads at low ball screw speeds and small preloads and high ball screw speeds.
It is an object of the present invention to provide a ball nut having adjustable preloading.
It is yet a further object of the present invention to provide a ball nut having adjustable preloading such that at high ball screw speeds, ball nut preloading can be made low while at low ball screw speeds, ball nut preloading may be made high.